1. Field of the Invention
The present invention relates to a battery pack used as a main power supply for a portable computer and a charge/discharge control apparatus therefor.
2. Related Art
A plurality of series-connected nickel-cadmium (Ni--Cd) or nickel-hydrogen (Ni--MH) secondary battery cells are generally used as a conventional main power supply battery for the functions of the above portable computer, a load increases, and user demand has arisen for prolonging a battery drive time. For these reasons, demand has increasingly arisen for a secondary battery having a larger capacity.
Under the above circumstances, a lithium-ion secondary battery can be used as a main power supply battery.
If eight lithium-ion battery cells are connected in series with each other as in the conventional arrangement so as to meet the needs for a large capacity, thereby constituting a battery pack, the following problem is posed. The upper limit of a terminal voltage of a cell itself of the lithium-ion battery is about 4.2 V, while its lower limit is about 2.7 V. The upper limit of the voltage range of the eight series-connected lithium-ion battery cells is 33.6 V, while its lower limit is 21.6 V. The voltage range is widened, and the absolute value of the voltage is increased, resulting in inconvenience. In addition, a conventional AC adapter is of 15 or 18 V, the voltage of the eight series-connected lithium-ion battery cells becomes about twice the AC adapter voltage. A circuit size becomes undesirably increased due to the circuit design of a protection circuit if the voltage exceeds 20 V. For this reason, three parallel circuits each consisting of three series-connected lithium-ion battery cells are prepared, and any one of the three series circuits is selectively used, as shown in FIG. 1. In this case, the voltage of one series circuit of the lithium-ion battery cells falls within the range of 15 V, and matching with the AC adapter can be improved. A lithium-ion battery (e.g., a lithium-ion battery of 3.6 V.times.1,000 mAh) has a higher internal impedance than that of the Ni--CD or Ni--MH battery. When a larger current flows through the lithium-ion battery, a battery voltage is reduced by an increase in current accordingly. A smaller current is required to extract a maximum energy stored in the battery. That is, as shown in FIG. 3, the product of time and ampere is kept unchanged in the Ni--Cd or Ni--MH battery. The energy extracted in 8-hour use at 2 A is equal to that in 2-hour use at 8 A. In the lithium-ion battery, however, as shown in FIG. 4, the energy extracted in 2-hour use at 8 A is smaller than that in 8-hour use at 2 A. For this reason, in the arrangement of series-connected lithium-ion battery cells, as shown in FIG. 1, the energy stored in the battery cells cannot be satisfactorily extracted. The lithium-ion battery tends to be exploded against overcharge. The battery voltage must be strictly monitored for each battery cell. In the arrangement shown in FIG. 1, a monitor circuit must be arranged for each lithium-ion battery cell so as to perform charge/discharge control. The circuit is complicated, and the circuit size is increased, resulting in impractical applications.
In the parallel circuit consisting of the three series circuits each having three series-connected lithium-ion battery cells, as shown in FIG. 2, when any one of the lithium-ion battery cells of each series circuit is broken due to short-circuiting or the like, the remaining two battery cells are charged with an energy of three lithium-ion battery cells, resulting in overcharge. To prevent this, a diode may be added to the above circuit. However, both a charge diode and a discharge diode must be added to each battery cell, and the circuit size is undesirably increased.
In a conventional charge circuit which can incorporate a plurality of single lithium-ion secondary battery cells, a plurality of battery sets thereof, or a plurality of battery packs using the single lithium secondary battery cells or lithium battery sets, a charge scheme shown in FIGS. 36A and 36B is employed.
More specifically, the secondary battery (Batt 1) of the first pack is charged with a constant current (constant current charge). When the battery voltage is increased and reaches a specified voltage, charge is continued with a constant voltage (constant voltage charge).
In a constant voltage charge state, the charge current is gradually decreased. A full charge state is set when the charge current becomes smaller than a specified value.
Upon detection of the full charge state of the secondary battery (Batt 1) of the first pack, the secondary battery (Batt 2) of the second pack is charged in the same manner as in the secondary battery of the first pack.
In the above conventional charge scheme, however, charge of the next battery pack is not started until the full charge state of the preceding battery pack is detected. For this reason, the total charge time of a plurality of battery packs becomes (Charge Time of Battery).times.(Number of Battery Packs).
In the constant voltage charge mode, the charge current of the lithium-ion secondary battery is gradually reduced due to its battery characteristics. Along with this decrease, the current supply capacity has a sufficient margin. In the conventional scheme, however, the power corresponding to the sufficient margin cannot be used for the next charge operation, thus wasting the power.
In a conventional scheme, in charging a secondary battery required for constant voltage charge, as shown in FIG. 9, first of all, charge is started with a constant current. When a battery voltage 5 in the charge ON state is detected to reach a target value 3, and charge is switched to charge with a constant voltage at a point A. A charge current 7 is then gradually reduced. Charge with a constant voltage continues until a voltage 1 of the battery itself becomes the same potential as that of the target value 3 and a charge current does not flow (i.e., until the state reaches a full charge state).
As described above, when the battery voltage in the charge ON state reaches the target value 3, charge with a constant current is switched to charge with a constant voltage. At this time, the charge state is not yet the full charge state because the battery voltage includes a voltage drop caused by an internal resistance and the like. A long period of time is required until the full charge state is obtained after charge is switched to charge with a constant voltage.
In a conventional battery set obtained by series-connecting lithium-ion secondary battery cells, variations in secondary voltages during charge is corrected as follows.
More specifically, when a battery set obtained by series-connecting two secondary battery cells is taken as an example, the half of the battery set voltage is input to an amplifier, and an output from the amplifier is supplied to a connection point between the series-connected secondary battery cells, thereby setting the output voltage of the amplifier to 1/2 the total voltage of the battery set. The output from the amplifier is connected to the connection point of the secondary battery cells through a current limit resistor. Therefore, a correction current flows due to a potential difference and resistance between the output voltage of the amplifier and that of the connection point between the secondary battery cells.
The following problem is posed in a battery voltage measuring means having the above conventional arrangement. When the battery voltage of each secondary battery cell in the battery set is measured outside the battery set, a value including a voltage drop caused by the internal current limit resistance in the battery set is measured, and an accurate battery voltage cannot be measured.